Computers and various computer related systems have been developed over the years. Advancements in these computers and computer systems occur rapidly, especially with the competitive nature of the commercial marketplace. These computers often include a display or monitoring screen for monitoring data, one or more processors for processing data, and a hard-disk drive for storing and retrieving data. These hard-disk drives often take the form of a magnetic storage device which is a portion of the "recording channel front-end." This front-end of the hard disk drive, for example, includes a read/write transducer which operates as a recording head, an electronics module which includes a read pre-amplifier circuit and a write driver, and interconnections between the various heads and the electronics module. These electronics modules conventionally are placed close to the heads to keep the interconnections as short as possible.
A read pre-amplifier circuit for hard-disk drives often includes a magneto-resistive ("MR") element as the sensor. The MR element advantageously does not present as much bandwidth restriction as some other types of read elements, e.g., inductive. The MR preamplifier circuit generally has a very low level "read" signal which is generated by an MR sensor and which is detected by a low noise preamplifier circuit. The preamplifier circuit of the electronics module generally has to provide the "sense current" for reading the resistance changes of the energetically passive MR sensor. In some types of MR preamplifiers, this sense current also provides the circuit biasing needed so that the sense current doubles as the bias current. The MR sensor, for example, can be biased by either maintaining a constant current through it or by applying a certain constant voltage across it.
In all circuits, the MR sensor, whose electrical equivalent is a simple resistor of nominal value R (typically in the order of 10 to 50 ohms), detects a magnetic read signal from the hard-disk by sensing changes in magnetic data flux. A magneto-resistive effect causes the equivalent resistor value to change by a small fraction of R, e.g., a few tenths of a percent, resulting in a variation of the voltage programmed to bias the head in its most linear operating range. For most MR sensors, the nominal direct current ("DC") bias voltage across the head is about 100 to 500 mV, and is a function of head manufacturing characteristics. The preamplifier circuitry therefore should provide for both low-noise, digital-to-analog ("DAC") programmable current biasing of the head and very low-noise amplification of its signal.
In addition, some multiplexing of the preamplifier input stage to different read heads should be provided, without compromising the noise performance. In most current-bias, current-sense architectures, where the head is directly biased by a current source controlled by a current-summing DAC and where the signal voltage is connected to a low input-impedance preamplifier input stage, two large external decoupling capacitors are usually required to eliminate the noise contributions from the bias current source itself and from the analog circuitry connected to the base (or the gate for MOS input stages) of the preamplifier input transistor. The need for offset removal within the preamplifier also calls for the use of a large capacitor which is, however, usually combined with one of the others so as not to increase external component count.
As an example, in FIG. 3 of the article titled "Read/Write Amplifier Design Considerations For MR Heads," by Klaassen et al., IEEE Transactions ON Magnetics, Vol. 31, No. 2 (March 1995), pp. 1056-1061, such an implementation is shown where a first external capacitor C decouples the gate of the input low-noise device, while a second external capacitor (not shown) is used to implement a low-noise current source I.sub.b. One drawback of this particular example is the fact that the low-noise current source I.sub.b is directly connected to the drain of the input transistor and severely limits the amplifier bandwidth which is another critical parameter of the circuit.